Solid state electronic switches are widely used in the automotive industry because of their high speed, small size, and light weight. However, such devices are characterized by low on-resistance, which is the resistance of the device to current flow. Therefore, a large current flow through the device due to a short circuit condition can irreparably damage the device.
For low on-resistance devices, the current through the device is limited only by the resistance and inductance of the wire during a short circuit condition. As a result, the current increases substantially in a relatively short amount of time. This is especially true if the wire length is very short. Typically, electronic switches are protected by a short circuit protection circuit. Short circuit protection circuits are limited in response time, however, due to current detection delay, turn off delay, and turn off transient time. As a result, there is a delay between the detection of an over-current condition and the current interruption of the switch. In other words, in some circumstances, the current may reach a very high value before the protection circuit takes effect. Further, the short circuit current is inversely proportional to the length of the shorted wire.
A short circuit current waveform 10 for an exemplary electronic switch having a maximum rated current of 60 amps, an on-resistance of 2 mΩ, and a trip current set at 100 amps is shown in FIG. 1. The waveform 10 is exemplary of an electronic switch that utilizes conventional short-circuit protection. A short circuit protection circuit monitors the level of the current 12 flowing through the device. Voltage 14 is the voltage across the device. The protection circuit generates a shutoff signal if the current level 12 exceeds the trip current level 16 of 100 amps. However, the protection circuit does not generate the shutoff voltage signal until the current level 12 is above the trip current level 16 for tens of microseconds. This delay is referred to as a detection delay 18. Subsequently, after the protection circuit generates the shutoff voltage signal 14, additional time elapses. This delay is referred to as a turn-off delay 20. Therefore, it can be seen that the conventional short circuit protection did not adequately protect the device from exposure to high current. Because the current increase rate di/dt is very high during a short circuit condition, it is possible for the current level 12 to exceed 750 amps, despite having a trip current level 16 of 100 amps.